A light emitting diode (“LED”) typically includes a diode mounted onto a die or chip, where the diode is surrounded by an encapsulant. The die is connected to a power source, which, in turn, transmits power to the diode. An LED used for lighting or illumination converts electrical energy to light in a manner that results in very little radiant energy outside the visible spectrum. In a typical LED, a significant portion of the current that is applied to the LEDs is subsequently converted into thermal energy.
In an LED light source, the heat generated by the lamp may cause problems related to the basic function of the lamp and light fixture. Specifically, high operating temperatures degrade the performance of the LED lighting systems. Typical LED lighting systems have lifetimes approaching 50,000 hours at room temperature; however, the same LED lighting system has a lifetime of less than 7,000 hours when operated at close to 90° C.
LEDs are utilized as light sources in a wide variety of applications. Specifically, LEDs may be used in track lighting applications. Track lighting is used to accent or highlight merchandise in such a way that it stands out from the rest of the products around it. Typically, track lighting provides approximately three times more light on a product than the general illumination in the area. In this application, extremely bright LED light sources are used, which produce very high lumens from a relatively small package. LEDs may also be used in sealed, enclosed light fixtures, where the enclosure prevents the possibility of introducing ambient air into the light fixture. In these applications, as well as other LED applications, there is a need to incorporate a cooling system to prevent overheating and to maintain optimum lumen output.
There are three mechanisms for dissipating thermal energy from an LED: conduction, radiation, and convection. Conduction occurs when LED chips, the mechanical structure of the LEDs, the LED mounting structure (such as printed circuit boards), and the light fixture housing are placed in physical contact with one another. Physical contact with the LEDs is generally optimized to provide electrical power and mechanical support. Traditional means of providing electrical and mechanical contact between LEDs and the light fixture provide poor means of conduction between the LEDs and external light fixture surfaces (such as die cast housing). One disadvantage of using a thermally conductive structure within the light fixture envelope is that it allows dissipation of heat into the enclosure, which is generally sealed. This effectively raises the ambient temperature of the air surrounding the LEDs, thus compounding thermal related failures.
Radiation is the movement of energy from one point to another via electromagnetic propagation. Much of the radiant energy escapes the light fixture through the clear optical elements (light emitting zones, lenses, etc) and reflectors, which are designed to redirect the radiant energy (visible light in particular) out of the light fixture according to the needs of the application. The radiant energy that does not escape through the lenses is absorbed by the various materials within the light fixture and converted into heat.
Convection occurs at any surface exposed to air, but may be limited by the amount of air movement near the emitting surface, the surface area available for dissipation, and the difference between the temperature of the emitting surface and the surrounding air. In many cases, the light fixture is enclosed further restricting airflow around the LEDs. In the case of an enclosed light fixture, heat generated by the LEDs is transferred by convection to the air within the enclosure, but cannot escape the boundaries of the enclosure. As a result, the air within the enclosure experiences a build up of heat, which elevates lamp and light fixture temperatures and may lead to heat related failures.
Better thermal management allows the LEDs to be driven at higher power levels while mitigating the negative effects on life and light output normally associated with higher power input levels. Benefits associated with effective removal of thermal energy from within the light fixture include improved lamp life, smaller (lower cost) package sizes, and improved lumen output. Accordingly, there is a need for a cooling system that may be incorporated in LED track light fixtures and enclosed LED light fixture applications to allow LED light fixtures to maintain optimum lumen output.